It is well known that liquid metal contains varying amounts of non-metallic constituents, i.e. gas and non-metallic inclusions, and that their presence may give rise to defects in finished products. Many procedures have been proposed for the removal of the gas and inclusions.
Liquid metal treatment prior to solidification processing is necessary for a variety of casting processes including, but not limited to, sand casting, permanent mould casting, high pressure die casting, direct chill casting, twin roll casting and the like for the purposes of grain refinement, melt cleanliness, homogeneous microstructure and homogeneity of chemical composition, dispersing and distributing of both endogenous and exogenous particles.
The existing methods for liquid metal treatment mainly include, mechanical stirring by an impeller, electromagnetic stirring, and some other methods like gas induced liquid flow.
Mechanical stirring by an impeller is a very simple way to treat liquid metals. It only provides moderate melt shearing around the impeller, but causes serious vortex in the liquid metal and serious turbulence near the liquid surface, resulting in severe entrapment of gas and other contaminants from the melt surface. There have been a number of approaches to address such problems.
U.S. Pat. No. 3,785,632 issued to Kraemer et al. discloses a process and an apparatus for accelerating metallurgical reactions. The process includes mechanical stirring at the boundary between the molten bath and the reactant, using a twin-impeller. A centrifugal force component is created when the apparatus starts stirring and causes different curvature towards the margin of the ladle which leads to the acceleration of chemical reaction between the molten metallic material and the reactants.
U.S. Pat. No. 4,743,428 issued to McRae et al. discloses a method of mechanical stirring of liquid metals for producing alloys. The process introduces an agitating device mainly to accelerate the dissolution of alloying elements and slow down the formation of dross.
U.S. Pat. No. 3,902,544 issued to Flemings et al. discloses a continuous process of treating liquid metals by mechanical stirring to obtain semi-solid metallic materials with non-dendritic primary solid. In this process three augers are introduced and located in three separated agitation zones. The augers are more effective compared to the twin blade impeller. The distance between the inner surface of the agitation zone and the outer surface of the auger is kept sufficiently small so that high shear forces can be applied to the materials in the agitation zones.
U.S. Pat. No. 4,373,950 issued to Shingu et al. introduced mechanical stirring by an impeller into direct chill casting process to purify aluminium. Aluminium melt is purified by using a mechanical stirring apparatus to break down dendrites at the interface between the liquid and the solid, and dispersing the impurity released from dendrites into the whole liquid.
U.S. Pat. No. 4,908,060 issued to Duenkelmann discloses a rotary device comprising a hollow shaft and a hollow rotor attached to the shaft for dispersing gas in molten metal. The device introduces inert gas from the top of the shaft and delivers a large volume of inert gas into the melt for degassing of liquid metals.
The inventions discussed above all involve mechanical stirring. They neither provide the high shear rate required for melt conditioning, nor avoid the problems of entrapment of gas and other contaminants from the melt surface.
U.S. Pat. No. 4,960,163 introduces a mechanical stirrer in direct chill casting for achieving fine grain structure and a partition to divide the space in the DC caster into a supply reservoir and a solidification reservoir for avoiding turbulence near the liquid surface in the supply reservoir without weakening the stirring in the solidification reservoir. A certain degree of grain refinement by this invention was achieved but the results were not consistent from batch to batch.
U.S. Pat. No. 6,618,426 issued to Ernst discloses a process of electromagnetic stirring to treat liquid metals. This process used multiple coils with different directions to reduce the turbulence near the liquid surface. However, the shearing rate by electromagnetic stirring is low and the cost of the apparatus is high.
WO 2010/032550 (Nippon Light Metal Co. Ltd) discloses a metal melt refiner for use in a ladling chamber. It is essentially a multi-blade stirrer for degassing and deslagging liquid metals. However it has very little dispersing and distributing power and the whole assembly is not suitable for direct incorporation in existing casting processes.
There are known a method and an apparatus for stirring molten metal in the vessel of the furnace by using an electromagnetic field. The inductor of the running magnetic field is positioned along the vertical wall of the furnace. The furnace contains the passageway for molten metal. The incoming stream of molten metal from the passageway into the vessel is directed mainly along a wall of the vessel. However, the apparatus and the system thereof fail to attain the object of as the intensity of the jet-mixing in the middle of the vessel is lower than along the walls thereof. Thus, for melting of solid metal in the middle of the vessel, additional mechanical-contact stirring is required. Also another way of stirring with the placing of magnetic beads within the molten metal which are then moved in a circular manner thereby stirring the liquid Another shortcoming, that limits the use of said method and apparatus, is the necessity of long-term stoppage of the furnace for dismantling of the inductor and for replacement of plates for removal of slag from the passageway.
In another prior art a furnace is known with a fixed pocket along an end of the furnace, underneath which the inductor is placed. The bottom of the pocket is located flush with the bottom of the furnace. Metal pumps along the pocket and comes in the vessel through a window in the wall of the vessel. The intensity of the stirring in the middle of the vessel is lower than on the sides of the vessel.
As per another prior art the aim of which is to provide an apparatus for stirring that does not require any substantial reconstruction of the melting furnace and which has to secure the effective jet-mixing of the molten metal in the vessel of the melting furnace. Stirring is achieved in the intermittent regime. The set aim is not reached, because the mass of the molten metal, which may be discarded into the vessel of the furnace in the form of a jet, cannot exceed the capacity of the pipe of the apparatus. Shortcomings of said apparatus are the laboriousness of the removal of slag from the pipe, and the complexity of travel of the pipe of the mechanical drive pump.
According to yet another prior art there is provided a rotary device for treating molten metal, wherein the combination of a chamber, outlets having a larger cross-section than the inlets and cut-outs in the roof and the base, results in both improved degassing and improved mixing of molten metal such that rotation speed can be reduced while maintaining the same efficiency of degassing/mixing, thereby extending the life of the shaft and rotor, or degassing/mixing times can be achieved more efficiently at the same rotor speed, providing an opportunity to reduce treatment time. However, the controlled regulation of the rotational speed in accordance with the viscosity of the molten metal and the dimensions of the chamber, outlets and inlets is a task of difficulties. The vortex formed in the liquid metal and serious turbulence near the liquid surface, result in severe entrapment of gas and other contaminants.
According to the yet another prior art, there is provided a vibrational fluidly stirring apparatus comprising a tank for accommodating fluid; a vibration generating portion containing a vibrator; a vibration absorbing member disposed between the tank and the vibration generating portion; a vibrating bar operationally connected to the vibration generating portion and extended in the tank; and a vibration vane attached to the vibrating bar, wherein the vibration absorbing member comprises a rubber plate or a laminate of rubber plate and metal plate. The performance of the system is depend on vibration absorbing member and the system also have a drawback of scattering the liquid to the outside of the tank as controlled regulation of the vibrational frequency is very difficult.
Current mechanical or electromagnetic stirring for treating liquid metals causes turbulence near the liquid surface which is harmful for most casting processes. Therefore, the stirring speed must be limited in order to achieve a relatively stable liquid surface, and consequently both effectiveness and efficiency of liquid metal treatment are compromised.
For the reasons stated above, which will become apparent to those skilled in the art upon reading and understanding the specification, there is a need in the art for a system and method for liquid metal treatment prior to solidification processing that is scalable and independent/compatible to new technology platforms, uses minimum resources that is easy and cost effectively maintained and is portable and can be deployed anywhere in very little time.
It would be advantageous, therefore, to provide a method and apparatus that can be readily applicable to existing casting processes and can provide intensive melt shearing while avoiding entrapment of gas and other contaminants from the melt surface as well as supply such sheared melt down stream by pressurising the liquid or semi solid slurry/feedstock required for downstream processing.